The present invention relates to viscous fan drives for driving radiator cooling fans, and more particularly, to such fan drives which are especially adapted for heavy duty applications.
Viscous fluid couplings have been used as fan drives (i.e., to drive the radiator cooling fan of an internal combustion engine) for many years. The primary reason for using a viscous fan drive, rather than a direct, mechanical drive of the fan, is to reduce the amount of horsepower consumed by the radiator cooling fan.
Initially, viscous fan drives were utilized on automobiles, and in more recent years, have also been used on light trucks. In either case, as is well known to those skilled in the art, a viscous fan drive is typically used only on a "north-south" engine, i.e., an engine having its axis oriented in the direction of travel of the vehicle.
Traditionally, larger vehicles such as class 8 trucks, being equipped with diesel engines, have utilized radiator cooling fans driven by an ON-OFF clutch, such as a pneumatically actuated friction clutch. In such a clutch, the output is driven either at the speed of the input ("ON"), or is disengaged and is not driven at all by the input ("OFF").
Although such ON-OFF clutches for heavy duty applications have been generally satisfactory, such clutches do have certain inherent limitations. It is now considered desirable by some of the vehicle manufacturers and fleet owners to be able to "modulate" the radiator cooling fan, i.e., at certain operating conditions, to operate the fan, but at less than full input speed, with the ratio of output speed to input speed being somewhat "controllable". Such modulation cannot be achieved readily with an ON-OFF clutch of the frictional engagement type. However, such modulation can be readily achieved with a viscous coupling.
It is also considered desirable by the vehicle manufacturers and fleet owners that the control of the fan drive be of the "open-loop" type, wherein the fan speed to input speed ratio is merely selected by a relatively inexpensive actuator, rather than being of the "closed-loop" type, wherein actual fan speed is monitored and used as an input to close the control loop. A viscous coupling is also especially adapted for such open-loop control.
Unfortunately, conventional viscous fan drives have not been especially well suited for heavy duty applications for several reasons. First, typical commercial viscous fan drives include a fluid reservoir which rotates at output speed, which is acceptable for most fan drive applications. However, for heavy duty applications it is especially desirable to provide a very low idle speed, and in a viscous fan drive, if the output member (and the reservoir) rotate very slowly, it is extremely difficult to achieve engaged operation of the fan drive, for reasons well understood by those skilled in the art.
Secondly, conventional commercial viscous fan drives have not been constructed in a way which would be suitable for driving the type of large, heavy fan required to cool the radiator in a heavy duty application. This is especially true of the conventional manner of mounting the prior art fan drive, which to attach its input shaft flange to a mating flange on a shaft extending out of the engine water pump. The result is that the fan drive is mounted in a cantilever fashion, thus constituting a substantial "overhung" load. Such a mounting arrangement is acceptable in the case of a small automotive fan drive, but is totally inadequate in the case of a much larger fan drive for a class 8 truck.